Abstract: The role of lattice oxygen in catalytic dehydrogcnation of ethylbenzene over industrial iron-oxide-based catalysts, 11^# (Cr-consaining) and 210^#(Cr-free), has been investigated by means of the following experiments: (1) Determination of amounts of H₂Oproduced due to removal of lattice oxygen by reaction with ethyl-benzene used as the feed without addition of steam, and observation of gradual decay of catalytic activities, and changes in the X-ray powder-diffraction patterns of the used catalysts; (2) with elhylbcnzcne plus D_{2Oas the feed, determination of the change in D2/H2 ratio in the exit gas for very high and very low space velocities; and (3) vvith ethylbenzene plus D2¹⁸Oas the feed, observation of isotopic exchange of lattice ¹⁶Owith steam ¹⁸Ofrom the red shifts in the IRspectra of the used catalyst. The results indicate that direct heterolytic dehydrogenalion of ethylbenzene appcared to be the major rcction pathway, but a minor reaction pathway of cthyl-benzcne dehydiogenation by oxygen-transfer mechanism also appeared to make an appreciable contribution (about 20%) to the overall conversion. Mechanisms of these two reaction pathways are discussed. For the oxygen-transfer dehydrogenation mechanism, the importance of electron transport between neighboring active-sites operating cooperatively in opposite phases of their redox cycles is pointed out.}